1. Field of the Invention
The present invention relates to a sheet processing apparatus having a cutting function, a sheet processing method, and an image forming system.
2. Description of the Related Art
Conventionally, an image forming system is known which has a post-processing apparatus (sheet processing apparatus) connected to an image forming apparatus, such as a copying machine and a laser printer, thereby having the capability of operating in a bookbinding mode for performing a bookbinding process. In this bookbinding mode, there are carried out a saddle stitching process for stitching sheets at a center thereof, and a folding process for folding sheets at a center thereof.
FIG. 46 is a view of a bundle of sheets subjected to a conventional saddle stitching and center-folding process. In a book-bound sheet bundle, when the number of sheets in the bundle is large or the thickness of each sheet in the bundle is large, the respective positions of opposite ends of folded sheets of the bundle are not aligned between the inner sheets and outer sheets, which degrades the appearance of the finished sheet bundle. As a solution to this, a post-processing apparatus has been proposed which is equipped with a cutting function for cutting off the ragged end of a sheet bundle by a width of several mm using, e.g., a cutter, to thereby align the respective ends of sheets for improvement of the appearance of the finished bundle (see Japanese Laid-Open Patent Publication (Kokai) No. 2003-341919).
Further, when cutting is performed, the ragged portions L are cut off (see FIG. 46). Therefore, from the viewpoint of operability, it is a method easy for users to understand to designate a length to be cut off from the ragged side end of a sheet bundle, and hence such a method of setting a cutting amount is conventionally employed (Japanese Patent Publication (Kokai) No. 2004-210509).
However, in the conventional sheet processing apparatus, there remains room to be improved in the following points:
Recently, elongated sizes which are slightly longer than respective predetermined standard sizes come to be used in printers, e.g., of copying machines. When sheets having such an elongated size are used, a margin is secured along the sides of each sheet, and an image having a predetermined size is formed in a central area of the sheet. The sheet having the image formed thereon is subjected to a cutting process for cutting off the margins along the sides of the sheet.
Thus, the user performs the cutting process on a bundle of book-bound sheets having an elongated size, not only for cutting off the ragged portions but also for cutting off the margins along the sides of the sheets.
Therefore, for a user who wants to cut off unwanted portions, such as margins, of sheets, according to the size of images formed on the sheets, there arises the following problems: When designating a cutting length (cutting width) from sheet end, the user is required to grasp the size of recording sheets, and the size of images to be printed thereon in advance, and then set a cutting amount by calculation. This makes an error in setting the cutting amount liable to occur, which degrades the user friendliness.